1. Field of the Invention
The present invention relates generally to a method and apparatus for improving transmit diversity in a broadband wireless communication system, and more particularly, to changing a preamble structure in a broadband wireless communication system using multiple antennas based on multiple input multiple output (MIMO) to improve transmit diversity.
2. Description of the Related Art
Wireless communication systems with multicarrier transmission techniques were first applied to military radio communications in the late 1950's. An orthogonal frequency division multiplexing (OFDM) technique, serving as a representative multicarrier transmission technique for overlapping orthogonal subcarriers, started in the 1970's. The OFDM technique converts a serially input symbol stream into parallel signals and modulates the parallel signals using a plurality of orthogonal subcarriers to transmit the modulated parallel signals. In view of orthogonal modulation between multiple carriers, the OFDM technique is limited.
In 1971, Weinstein, et al. proposed that OFDM modulation/demodulation can be performed efficiently using discrete Fourier transform (DFT). The introduction of a guard interval further reduced adverse effects of multipath propagation and delay spread in an OFDM system. Although hardware complexity was an obstacle to OFDM system implementation, recent advances in digital signal processing technology, including fast Fourier transform (FFT) and inverse fast Fourier transform (IFFT), make it possible to implement the OFDM system.
As a result, OFDM has been widely used for digital data communication technologies, such as digital audio broadcasting (DAB), digital TV broadcasting, wireless local area network (WLAN), and wireless asynchronous transfer mode (WATM). OFDM has efficient frequency use and reduces the effects of intersystem interference (ISI) by using guard intervals. Because OFDM is robust against multipath fading, it can provide optimum transmission efficiency when high-speed data is transmitted.
Multiple access techniques based on OFDM are usually classified as a orthogonal frequency division multiple access (OFDMA) technique or a frequency hopping (FH)-OFDM technique. An example of OFDMA in a broadband wireless communication system is the well-known Institute of Electrical and Electronics Engineers (IEEE) 802.16d or 802.16e system. The IEEE 802.16 standardization group is currently creating the IEEE 802.16d/e standard for providing wireless broadband Internet service to a fixed or mobile terminal through broadband wireless communication systems such as OFDMA, etc.
The OFDMA system can assign a plurality of subchannels configured by different subcarriers to different users. When an adaptive antenna system (AAS) is used to improve system capacity in the OFDMA system, a multiuser diversity gain can be obtained. When a multiple input multiple output (MIMO) system is used as a representative example of the AAS, information is spatially multiplexed and then transmitted, such that communication system performance is improved significantly.
A system for transmit diversity through multiple antennas in a cellular base station based on the OFDMA or OFDM technique has a relatively simple transceiver structure and a high diversity gain. The transmit diversity is included as an option in an IEEE 802.16d system. Accordingly, a preamble structure is included so that a receiver can discriminate signals received from a number of transmit antennas. However, only preambles for two transmit antennas are provided in the prior art.
The introduction of a base station with four or more antennas is required to obtain a high diversity gain. Conventional methods, therefore, are limited for maintaining orthogonality between antennas on the basis of frequency division.